Another SQL newbie Question!

Hi! I have just started on SQL in MSAccess, trying to make an Order database.

My question is this (trying to keep it as simple as I can!):

I have three tables Category, TableA and TableB:

The category table has an ID and the category name.
TableA has a Category field (linked to the category table) and a Description field (and an ID field).
TableB also has a Category field and a Description field.

I want to be able to enter data into the Category field of TableB, and then have the Description field as a dropdown with only Descriptions that are in the Category named.

I thought this would be simple, and I had something like:
SELECT TableB.Category, TableA.Category, TableA.Description
FROM TableB,TableA
WHERE TableB.Category=TableA.Category;

Now I believe that from the error (error in FROM clause), you cannot get data from the table you are currently using.

I hope this makes some sense to somebody. It's easy to get confused and end up going in circles trying to find a solution!

Any help would be really appreciated